A Study of the Catalytic Hydrogenation of Methoxybenzenes Over Platinum and Rhodium Catalysts

[From the Summary]

The catalytic hydrogenation of methoxybenzenes and other aromatic methoxyl compounds has been studied. Hydrogenation of the aromatic ring was usually accompanied by some cleavage of the methoxyl groups. This cleavage occurred predominately upon the carbon-oxygen bond adjacent to the aromatic ring. The cleavage reaction was found to occur only as a part of the aromatic hydrogenation process. The amount of cleavage was found to be dependent upon the catalyst used. Platinum led to extensive methoxyl cleavage while rhodium catalyzed ring saturation with little cleavage. Also found to influence the extent of cleavage were reaction temperature and acid concentration.

A mechanism for the cleavage has been proposed. This mechanism involves the formation of a methoxycyclohexene or equivalent in which the carbon-oxyggen bond of the ether is labilized by the β - δ carbon-­ carbon double bond.

The utility of various rhodium compounds as catalysts for the hydrogenation of the benzene nucleus has been demonstrated. With respect to catalytic activity, supported catalysts have been shown to be superior to the pure metals when the two groups are compared on the basis of percentage of active constituent.

The effect of substitution and number of substituents on the rate of hydrogenation was found to be about the same for the various methoxybenzenes as for the corresponding methyl compounds.